I've seem some larger projects at bigger organizations where extensive wikis were available. I'm pretty sure updating docs/wikis was part of the "definition of done" on a task. Problem was, most of the devs were terrible, terrible writers. There was little to no consideration made for "explain this as if I don't already understand it" on these pages. They were dry, full of useless generated ERDs and other generated filler, and generally I'd end up reading the source code or reading the integration tests to get a better sense of what was going on.

All in all, I actually agree with you. Knowledge bases are great when done right. But it takes time and a general knack for writing, and that's not something that most devs have from my experience.